The present invention generally relates to a sewing device for producing a stitch row having fastening stitches and tack stitches, which, in particular, are of a bar tack configuration produced by means of zig-zag stitching.
In the U.S. Pat. No. 3,983,825 there is described a method and an apparatus for sewing a pocket cut on a workpiece, at which both pieces are clamped by a workpiece clamping mechanism and relatively moved to the needle of a sewing machine performing both, fastening and tack stitches. Due to different feed directions of the workpieces in relation to the stitch forming elements such as needle and looptaker, there are unconstant conditions as the needle thread and the looptaker thread are pulled off, which causes a variation of the thread tensions, i.e. a well appearing stitching for decorative applications can't be produced.
In order to eliminate the described problem, a method and an apparatus for forming normally appearing stitches was published in the U.S. Pat. No. 3,827,382, at which the value of the needle-thread-tension is altered depending on the direction of the relative feed motion between the workpiece and the stitch forming elements.
The above cited sewing devices operate with a continuous feeding movement which in addition to the variation of thread tension causes difficulties. Depending on the feed direction the needle is exposed to differently directed lateral forces acting on the needle, which effects the loop formation of the needle thread as well as the action of the looptaker and also creates problems as to promote the formation of a burr at the needle and/or wearing the beak of the looptaker due to collision of both elements.
In the U.S. Pat. No. 4,088,085 it is proposed to equip such type of sewing device with a movable needle guide as to eliminate the described problem connected to wear of the needle and/or looptaker, which reduces the productivity of such sewing device. The described needle guide requires an expensive and complicated drive mechanism and a special control on one hand and only protects needle and looptaker against collision on the other hand. In the operation of producing stitches in stiffer materials such as leather or plastic the described needle guide causes an elongation of the hole in the material being perforated by the needle as the workpiece is exposed to additional stresses. Since the dimension of workpiece feed as the needle penetrates the workpiece is proportionally to the stitch length, the producable stitch length at such sewing devices is restricted to a value of about 2.5 to 3 mm.
According to the U.S. Pat. No. 4,157,686 a sewing device of the described kind may be equipped with a special drive system performing the feed movement of the work holder in relation to the needle of the sewing machine, at which the feed movement is of intermittent character as to eliminate lateral forces deflecting the needle while penetrating the workpiece and thus eliminating sewing problems. The proposed work holder drive system requires a special planetary gear, which either is free of any back lash or liberated of any back lash by means of additional components. Due to the superimposition of a continuous and an oscillating movement the total mechanical structure of the sewing device is exposed to vibrations, which finally interfer with the movements of the work holder, so that the lateral needle deflection can't be totally eliminated.